I remember the first time I had my legs swept out from under me..I was like 14 or 15 and playfighting /sparring with a Karate guy outdoors and he just did a really quick backspin from a crouched position and swept my legs. It freaked me right out!Luckily it happened to me when I was young so I was wary of it early on..
As for smiling..I start grinning when someone amps up on me or some noob tries to prove himself with me ..its a sign that I am ready to crank it up as well.

I'll smile when the match is heating up, or to encourage the other person to come at me, when their nervous.

Personally, if I spar with someone that has control issues I play a pretty defensive game, only tagging clear shots when I know I can get out of the way of counter shots easily... but I am typically much smaller than everyone else and don't want to get hurt. I try to keep the intensity low so everything stays as controlled as possible. Trying to match power and aggressiveness with someone that "spazzes" only keeps that anxiety there which makes them unable to control themselves. I really think people hit too hard or go crazy because they're nervous, not because they're trying to prove how hard they can hit or because they're an asshole.

It's pretty much always better to try and keep things at a steady, controlled pace where both you and your partner can practice solid fundamentals... at least as far as improving your game goes. Particularly in instances like this where you really want to "teach" your partners how to properly spar, it's good to calm things down.

edit: and I'd also recommend not smiling at people that may or may not understand you're not just being a smug jerk :P

When all of a sudden I do really, really well in my perception because I'm used to getting pwnt, I'm like, "Holy crap, where did this come from?"

IMO it's better to think you suck than to think you're good. You don't want to be over estimating yourself if you ever end up in a self defense situation.

This is a difference between "sport vs street" really. To be a successful fighter you need to have a pretty high opinion of your own skills. Your ego and unwillingness to have anything get in the way of feeling like you're the best is the most important thing to hold onto preparing for and being in a fight. In a self defense scenario of course it's about self preservation, not winning or ego or being the champ, so I suppose it's a different sort of mentality. I only do martial arts for competition reasons, so self defense isn't something I take into consideration in my training.

Well, that was very helpful. I've only been doing Muay Thai for a few months now and I feel like I'm starting to grasp some of it. Although, naturally, the biggest improvement can only be achieved via training/ sparring, reading threads like this one can help get in a better mindset for it and point one to or remind one of things that haven't been so clear before.

Thanks for the info on sparring. What you said makes perfect sense. Two guys from my gym fought this weekend and in our last sparring session I was taken by surprise, the intensity was alot higher than ever before and was an eye opener for me because I am working towards getting my first fight. It was however a great experience to see where i need to be with my conditioning and skill level before I get in the ring.